There are known processes of the general kind above-referred to which the sludge and other material of that type is mixed with a flocculating agent and then conveyed through a filtering device for separating the filtrate from a cake as formed from the sludge upon extracting excess liquid therefrom.
To effect continuous dewatering of sludge, sewage and similar material after preceding total flocculation by addition of polyelectrolytes or other suitable flocculents centrifuges and stationary pressure filters are used. Also used are filtering devices which include as filter material sieves or textile fabrics, for instance, filter belt presses or filter presses of the drum type. There are also known for the purpose vacuum drum filters or vacuum flat bed filters and combinations of several such assemblies. To obtain the desired low residual moisture in the finished product it is generally required to carry out the removal of moisture in several stages when devices of the kind heretofore known are used.
In the preliminary dewatering zone of devices as now known, the flocculated suspension is subjected to a relatively low pressure drop; in most instances merely the force of gravity is applied or a relatively low hydrostatic pressure.
In the subsequent zone the pressures or other forces which are used to effect dewatering must be considerably higher since the preliminary dewatering already results in a corresponding hardening of the material to be processed. In the third zone, the pressure to be applied must be further increased whereby simultaneously shear forces must be applied by causing the compressed cakes to move.
In particular, the pressure and shear forces as are required in the last two zones causes a considerable accumulation of solid material on and in the filter drums or in the fabric bands used as filters. As a result, such filter material cannot be used for dewatering at low pressure. In continuous operation, it becomes necessary to operate a washing device either permanently or at intervals--using water at pressures from and to eleven atmospheres
Such washing devices serve to flush out solid particles as are retained in the meshes of the fabric used as filter material. The quantity of water used for washing is considerable in practice and may reach several times the quantity of sludge to be processed. Moreover, the wash water tends to be strongly contaminated by the solid particles. In some cases up to 10% of the solid material may be carried out with the wash water; another 5% may be in the filtrate water from the higher force zones of dewatering. These solids often impose great demands on other processes within the system; their need for separate additional purification creates significant additional costs.
The output of filter belt presses, vacuum filters, etc. is frequently limited by the efficiency of the preliminary dewatering, at the end of which the sludge or other material to be processed should be of a consistency such that it could be subjected to forces greater than atmospheric. A typical case is the clarification of sewage in installations as used by communities, in which flocculation is effected by mixing of the sludge or sewage with polyelectrolytes. Large slimy pieces tend to form which move slowly over the sieves or the filtering fabrics and such large pieces constitute a dense layer which impedes the desired formation of cakes. In some instances it prevents further filtration. By turning over such masses the situation can be remedied to some extent, but there always remains a certain amount of clogging on the filtering medium. This fact and condition adds signficantly to the cost of the dewatering installation itself, to its subsequent operating and service costs. and to the size of buildings required for its installation.